function* fetchData() {
    // 性能
    let res1 = yield (ajax(url, () => { }))
    let res2 = yield (ajax(url1, () => { }))
    let res3 = yield (ajax(url2, () => { }))
}

let it = fetchData(); // Generator
let result1 = it.next();
let result2 = it.next();
let result3 = it.next();

